The present invention relates to a process for preparing printing plates and, more particularly, to a process for preparing printing plates without the application of wet developing agents.
Processes for forming printing plates are well known in the art. Typically, they involve exposing a plate which is uniformly coated with a photopolymerizable composition to actinic radiation through a stencil or negative. In the exposed areas the photopolymerizable composition reacts and forms water or solvent insoluble material bonded to the surface of the plate. After exposure, the portions of the photopolymerizable layer which have not been exposed to radiation are washed away to reveal the underlying surface of the plate.
In the more common printing plates, the photopolymerizable material is a water soluble, light-sensitive diazo resin which is carried on an aluminum base sheet prepared in accordance with the teachings of U.S. Pat. No. 3,181,461 to Fromson. Upon exposure, the diazo resin reacts to form a water insoluble, hydrophobic material. After exposure the plate is washed with water to remove the unexposed water soluble diazo resin and thereby expose the surface of the aluminum base sheet in those areas. The exposed surface of the aluminum base sheet absorbs water and rejects ink whereas the photohardened diazo resin rejects water and accepts the ink. The plate can be used in planographic or lithographic printing.
While photographic processes for making printing plates have been used in the art for several decades and while numerous processes have been investigated, there is still a need for a dry process for preparing printing plates--that is a process which does not require the application of solvent or water baths to develop the printing plate by selectively removing portions of the photosensitive layer.
While dry processes have been known, they have not been completely satisfactory. For example, processes employing stripping development are known wherein a photopolymerizable substratum provided on a carrier sheet is selectively adhered to a printing plate support by exposing it to radiation in contact with the support. Upon peeling the carrier sheet away from the support following exposure, and in some cases heating the support, the substratum remains on the carrier sheet in the unexposed areas and adheres to the support in the exposed areas. As will be readily appreciated these systems are difficult to design since materials must be selected such that the degree of tack or adhesion of the exposed substratum for the printing plate is greater than its adhesion for the carrier sheet in the exposed areas but less in the unexposed areas.